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Ants thrived in a cooling climate by taming fungus

By the time humans began the transition from hunting and gathering to agriculture about 10,000 years ago, colossal and sophisticated agricultural operations had been prospering under their feet for tens of millions of years.

Now a team of scientists from the Smithsonian Institution’s National Museum of Natural History have identified when and where the planet’s first farmers – ants – made a revolutionary leap in their own agricultural evolution, developing farming practices rivalling modern human agriculture by domesticating fungal crops to the point where the fungi’s survival and evolution became dependent on their formic farmers.

The transition to this higher form of ant agriculture from a lower, “primitive” practice, which involved subterranean farming of fungi that also existed in the wild, occurred about 30 million years ago, following the Terminal Eocene Event when the planet’s climate cooled considerably, according to the new research, published today in Proceedings of the Royal Society B.

The findings, using phylogenics – the analysis of complete genome sets to identify evolutionary events – overturn prevailing theories that the transition took place in the same habitat where ant farming began: the rainforests of South America, where fungus-farming ants (known as the Attini tribe) originated shortly after the Cretaceous-Paleogene mass-extinction event around 60 million years ago, believed to have been caused by a meteorite impact.

It was during the “nuclear winter” aftermath of the impact, when relying on food sources not requiring immediate photosynthesis became a better path for survival, that ants feeding on certain fungi lost their ability to synthesise arginine, a crucial amino acid for producing proteins, and became dependent on the fungi for it, “committing them to agrarian life”.

Though this initial transition occurred in the humid South American rainforest, the authors found that “the ancestors of most major attine lineages, including the ancestors of higher fungus farmers and leaf-cutter ants, probably evolved in dry or seasonally dry habitat”. They hypothesise that moving into dry habitats may have been the trigger causing the farmed fungus to diverge from its wild relatives.

The scientists, led by entomologist Ted Schultz, the Smithsonian museum’s curator of ants, postulate that the wet conditions of rainforests conducive to “wild” fungi limited the dependence of fungal crops on their ant farmers. As the climate cooled and drier habitats became more common, however, cultivated fungi became dependent for survival on the climate-controlled conditions of underground agricultural ant colonies, and unable to escape and mix it up with wild fungi. “If things are getting a little too dry, the ants go out and get water and they add it,” Schultz explains. “If you’ve been carried into a dry habitat, your fate is going to match the fate of the colony you’re in.”

Schultz argues that studying the co-evolution of ant farmers and their fungal partners may offer important lessons to inform our own challenges with agricultural adaptation to a changing climate. “These higher agricultural ant societies have been practicing sustainable, industrial-scale agriculture for millions of years,” he says. “[It] provides all the nourishment needed for their societies using a single crop that is resistant to disease, pests and droughts.”